The issue of selectivity has been the most challenging aspect of ethylene oligomerization since its discovery.[1] In the last three decades, trimerization systems with high selectivity have been discovered, [2] but tetramerization catalysts with high selectivity remain elusive. [3] Central to the future success of this endeavor is to understand the factors responsible for the selectivity of the catalytic cycle. [4] Trivalent chromium complexes are the catalyst precursors that are most commonly used for these transformations. In the presence of alkyl aluminum activators, these species are reduced to the divalent state (responsible for nonselective oligomerization and/or polymerization) or the monovalent state (responsible for selectivity).[5] To complicate the scenario, there is also the possibility for these monovalent and divalent species to undergo disproportionative redox processes that give inactive zero-valent chromium together with higher-valent species that are readily available for further cycles of reduction/reoxidation.[6] Because of the presence of such a redox dynamism in the catalytic cycle, the ancillary ligand system determines the selectivity by preferentially stabilizing one particular oxidation state. For example, when highly reactive monovalent species are provided with a sufficiently long lifetime, selective oligomerization is initiated by the so-called redox reaction/ring-expansion mechanism. [2,5,7] As mentioned above, selective ethylene tetramerization remains exceedingly rare. With a maximum of around 77 % in the case of the process developed by SK Energy,[3d] the selectivity is definitely good, but still far from the levels obtained with the trimerization systems. In addition, the unavoidable formation of polymers poses serious reactorfouling problems that complicate industrial application. The same redox reaction/ring expansion mechanism that accounts for the selectivity of the trimerization cycle has also been invoked for the rationalization of the tetramerization. [3b, 8] Following this mechanism, it is conceivable that high selectivity cannot be reached. The selectivity in this mechanism is determined by the rate of the reduction/elimination step compared to the rate of further ring expansion. If the sevenmembered ring is capable of expanding readily into the ninemembered ring, it is hard to imagine why additional expansion should not occur equally fast. [9] In the end, a distribution of oligomers is to be expected and 1-octene may be a dominant product.Rosenthal and co-workers [10] first emphasized this problem and postulated an alternative mechanism for the highly selective formation of 1-octene. According to their hypothesis, a dimetallic system with two low-valent chromium centers that are not linked with each other may independently form two five-membered metallacycles. Cooperative dimetallic reductive elimination selectively affords 1-octene. It is by following this fascinating hypothesis that we have recently for the first time observed the formation of 1-octene that was un...

Going one, twice…︁ Reaction of the mononuclear complex [{η5‐(tBu)2C4H2N}CrCl2(thf)] with AlEt3 afforded the dinuclear species [{[η5‐(tBu)2C4H2N]CrEt}2(μ‐Cl)2] (see picture; Cr purple, Cl green, C sticks/gray, H white). The complex acts a single‐component selective trimerization catalyst; a higher loading of AlEt3 activator afforded isomerization of 1‐hexene to cis, trans 2‐hexene.

Reaction of the divalent [(t-Bu)NP(Ph)(2)N(t-Bu)]CrCl(2)Li(THF)(2) (1) with 1 equiv of vinyl Grignard (CH(2)=CH)MgCl reproducibly afforded the triangulo {π-[(t-Bu)N-P(Ph)(2)-N(t-Bu)]Cr}(2)(μ,μ',η(4),η(4)'-C(4)H(4)){σ-[(t-Bu)N-P(Ph)(2)-N(t-Bu)]Cr} (2) containing a σ-/π-bonded butadiene-diyl unit. The diene-diyl moiety was generated by an oxidative coupling and deprotonation of two vinyl anions. The crystal structure revealed that of the three chromium atoms, each bearing one NPN ligand, two are perpendicularly bonded to the two sides of the π-system of the butadiene-diyl residue in a sort of inverted sandwich type of structure. The third is instead coplanar with the doubly deprotonated C(4) unit and σ-bonded to the two terminal carbon atoms. Despite the appearance as a Cr(II)/Cr(I) mixed valence species, DFT calculations have revealed that the structure of 2 consists of three divalent chromium atoms, while the additional electron resides on the π-system of the bridging organic residue. Complex 2 behaves as a single component selective catalyst for ethylene trimerization.

Two catalytic systems based on anionic ligands with the NPN structural motif, bearing tri-and pentavalent phosphorus, respectively, have been compared vis-a-vis their ability to selectively trimerize ethylene. In the case of the trivalent phosphorus ligands, reaction of the Cr(II) catalyst precursor [(t-Bu)NPN(t-Bu)] 2 Cr (1) with 2 equiv of MeLi afforded the Cr(III) species [(t-Bu)NP(Me)N(t-Bu)] 2 CrLi(OEt 2 ) (2). The same reaction with 3 equiv of MeLi yielded instead the Cr(II) species {[(t-Bu)NP(Me)N(t-Bu)]Cr(μ-Me)} 2 {Li(THF)} 2 (3). The P atoms of both 2 and 3 have been methylated. Activation of 2 and 3 with MAO produced an S-F distribution of oligomers. Conversely, activation of 3 with EtAlCl 2 exclusively afforded 1-hexene together with a small amount of polymer as a byproduct. Treatment of 2 with EtAlCl 2 did not yield an active catalyst. The reaction of CrCl 2 (THF) 2 with [(t-Bu)NP(Ph) 2 N(t-Bu)] À Li + , containing pentavalent phosphorus, afforded the Cr(II) derivative [(t-Bu)NP(Ph) 2 N(t-Bu)]Cr(μ-Cl) 2 Li(THF) 2 (4). Its alkylation with EtLi gave the ethyl-bridged dimer {[(t-Bu)NP(Ph) 2 N(t-Bu)]Cr(μ-Et)} 2 (5), which, upon thermolysis, afforded [(t-Bu)NP(Ph) 2 N(t-Bu)] 2 Cr (6). The structures of 5 and 3 are closely related, having in common two bridging alkyls and the same metal oxidation state but differing with respect to the P atom oxidation state and the presence/absence of alkali-metal cations. The catalytic behavior instead is remarkably different. Complex 5 is active as a self-activating selective ethylene trimerization catalyst, while complex 3 requires activation.

Kleine Änderung mit großer Wirkung: Die Einführung einer zusätzlichen CH2‐Gruppe in die Brücke von Liganden mit zwei P/N‐Einheiten führt zu unterschiedlichen Selektivitäten der entsprechenden chrombasierten Katalysatoren. Während 1 zu einem Ethylentrimerisierungssystem führt, ergibt 2 ein Ethylentetramerisierungssystem, das 1‐Octen mit hoher Reinheit sowie wenige bis keine Polymer‐Nebenprodukte produziert (siehe Schema; DMAO=Me3Al‐armes Methylaluminoxan).